How do I write TCP/UDP clients and servers?

In this guide, we will explore how to write TCP and UDP clients and servers in C++. TCP (Transmission Control Protocol) is a connection-oriented protocol that guarantees delivery of packets, while UDP (User Datagram Protocol) is a connectionless protocol that does not guarantee delivery, order, or error checking.

TCP Client Example

#include <iostream>
#include <cstring>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <unistd.h>

int main() {
    int sock;
    struct sockaddr_in server;
    char *message;
    char server_ip[] = "127.0.0.1"; // Localhost
    int port = 8080;

    // Create socket
    sock = socket(AF_INET, SOCK_STREAM, 0);
    if (sock == -1) {
        std::cerr << "Could not create socket" << std::endl;
        return 1;
    }

    // Setup server structure
    server.sin_family = AF_INET;
    server.sin_port = htons(port);
    server.sin_addr.s_addr = inet_addr(server_ip);

    // Connect to server
    if (connect(sock, (struct sockaddr *)&server, sizeof(server)) < 0) {
        std::cerr << "Connect failed" << std::endl;
        return 1;
    }

    // Send data
    message = "Hello Server";
    send(sock, message, strlen(message), 0);
    std::cout << "Data sent: " << message << std::endl;

    // Close socket
    close(sock);
    return 0;
}
    

TCP Server Example

#include <iostream>
#include <cstring>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <unistd.h>

int main() {
    int sockfd, client_sock, c;
    struct sockaddr_in server, client;
    char buffer[2000];

    // Create socket
    sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd == -1) {
        std::cerr << "Could not create socket" << std::endl;
        return 1;
    }

    // Setup server structure
    server.sin_family = AF_INET;
    server.sin_port = htons(8080);
    server.sin_addr.s_addr = INADDR_ANY;

    // Bind
    if (bind(sockfd, (struct sockaddr *)&server, sizeof(server)) < 0) {
        std::cerr << "Bind failed" << std::endl;
        return 1;
    }

    // Listen
    listen(sockfd, 3);
    c = sizeof(struct sockaddr_in);

    // Accept connection from client
    client_sock = accept(sockfd, (struct sockaddr *)&client, (socklen_t*)&c);
    if (client_sock < 0) {
        std::cerr << "Accept failed" << std::endl;
        return 1;
    }

    // Receive data
    recv(client_sock, buffer, 2000, 0);
    std::cout << "Data received: " << buffer << std::endl;

    // Close sockets
    close(client_sock);
    close(sockfd);
    return 0;
}
    

UDP Client Example

#include <iostream>
#include <cstring>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <unistd.h>

int main() {
    int sock;
    struct sockaddr_in server;
    char *message;
    char server_ip[] = "127.0.0.1";
    int port = 8080;

    // Create socket
    sock = socket(AF_INET, SOCK_DGRAM, 0);
    if (sock == -1) {
        std::cerr << "Could not create socket" << std::endl;
        return 1;
    }

    // Setup server structure
    server.sin_family = AF_INET;
    server.sin_port = htons(port);
    server.sin_addr.s_addr = inet_addr(server_ip);

    // Send data
    message = "Hello UDP Server";
    sendto(sock, message, strlen(message), 0, (struct sockaddr *)&server, sizeof(server));
    std::cout << "Data sent: " << message << std::endl;

    // Close socket
    close(sock);
    return 0;
}
    

UDP Server Example

#include <iostream>
#include <cstring>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <unistd.h>

int main() {
    int sock;
    struct sockaddr_in server, client;
    char buffer[2000];
    socklen_t client_len = sizeof(client);

    // Create socket
    sock = socket(AF_INET, SOCK_DGRAM, 0);
    if (sock == -1) {
        std::cerr << "Could not create socket" << std::endl;
        return 1;
    }

    // Setup server structure
    server.sin_family = AF_INET;
    server.sin_port = htons(8080);
    server.sin_addr.s_addr = INADDR_ANY;

    // Bind
    if (bind(sock, (struct sockaddr *)&server, sizeof(server)) < 0) {
        std::cerr << "Bind failed" << std::endl;
        return 1;
    }

    // Receive data
    recvfrom(sock, buffer, 2000, 0, (struct sockaddr *)&client, &client_len);
    std::cout << "Data received: " << buffer << std::endl;

    // Close socket
    close(sock);
    return 0;
}